Abstract

Abstract

The efficiency of nitrate use by turfgrasses is likely related to its efficiency of absorption by roots and its rate of metabolism in roots and shoots. This study was conducted to quantify the relationship between nitrate uptake rate and nitrate reductase activity with N use efficiency in a cool-season turfgrass. Six cultivars of Kentucky bluegrass (Poa pratensis L.), which differ markedly in field performance, were used to measure intraspecific variation in nitrate uptake, in vivo nitrate reductase activity of roots and leaves, and N use efficiency expressed as clipping mass per unit N in clippings. Companion field studies compared N use efficiency and metabolism among 14 Kentucky bluegrass cultivars established on an Enfield silt loam (Coarse loamy over sandy skeletal, mixed, mesic, Typic Dystrochrepts). Nitrate uptake rate was determined by an in situ nitrate depletion method. Nitrate reductase activity was assayed by an optimized in vivo method. Significant differences among cultivars were observed for nitrate absorption, nitrate reductase activity in roots and leaves, and N use efficiency. Ambient nitrate concentrations influenced these parameters and their intraspecific differences. Nitrate uptake and reduction were saturable at external nitrate concentrations in excess of 1 mM. Regression analyses demonstrated that nitrate reductase activity in roots and leaves was strongly influenced by nitrate uptake rate. Nitrogen use efficiency was negatively related to ambient nitrate levels, nitrate uptake rate and nitrate reductase activity, with nitrate reductase activity in leaves having the strongest negative effect on use efficiency. These results suggest that the efficiency of N use by Kentucky bluegrass may be increased by genetically altering nitrate reductase activity and its partitioning between roots and shoots.